Optical square.



Vio

i NTED STATES PATENT OFFICE.

OTTO EPPENSTEIN,

OF JENA, GERMANY, ASSIGNOR T0 TBE FIRM OF CAIBJ: ZEISS, 01"

JENA, GERMANY. v

OPTICAL SQUARE.

Specification ofgLetters Patent. Patented June 13, 1911.

Application filed March 15, 1910. Serial No. 549,555.

ifo all whom it ma/ z/ concern;

Be it;` known that I, OTTO EPPENSTEIN, a

citizen of the German Empire, residing at Carl-Zeiss strasse, Jena, in the Grand Duchy of Saxe-VVeimar, Germany, have invented a new and useful Optical Square, of which the following is al specification.-

Theinvention consists in an improvement in optical squares, the two reiiect-ing bodies of which are fixed to an intermediate member and consist each of a silvered t@dass plate. When such optical squares are in a condition, where they receive heat from without or spend heat outward, the reflecting surfaces undergo-even when the coeflicients of expansion of the .metallic intermediate memberand the glass plates are equal-as the observation of the reflected images Has shown, a deformation, which varies according to the manner in which the plates are fixed to -the intermediate member. When such an optical square forms a constituent part of a measuring instrument, a telemeter for example, this instrument is as longderanged as the condition stated lasts. According to the investigations, from which the present invention has taken its origin, there must be regarded as the cause of the -deformation a considerable difference between the mean .temperatures of the 1nter' mediate member on'one side and of' the plates on the is to be regarded as the elfect of the circumstance that the thermal conductivity of themetallic intermediate memberis a high multiple of the thermal conductivity of the glass plates. 1

T he invtutio-n consists in so selecting the material for the intermediate member that the thermal conductivity of the. latter is about equal to that of the. plates. According to the actual relation between the dimensions of the intermediate member and the size ot' the plates, the mentioned difference of the mean temperatures is found to be more or less reduced. Among the materials with the low thermal conductivity f tween the mirrors being Lsupposed to be "the other side, and this difference' of the glass and with suiiicient strength, the glass itself should be taken notice of. An intermediate member of glass can even be made integral. with the twoplates.

In the drawing some constrctional examples of the invent-ion are represented by All of them areoptical the angle bein the first four figures ahdm135 in Figure 5. i

In `theo optical square according to Fig. 1 the intermediate member a is a casing with threeopenings, the front one A of which perspective views. squares defiecting by 90,

serves for the entrance and exit of light,

whereas the two lateral ones are closed by silvered glass plates b and o cemented on the casing. j 1 i When the lower part of the hollow' space 'of the optical square is rounded so as to correspond to a cylinder of rays, the broken axis of which is shown, and when at the same time its covermg wall is omitted, the' optical square according to Fig.` 2 results.

. Thelexample of Fig. 3 represents an optical square, which is open also at the bottom. The intermediate memb'r a consists only in a hinder cross piece, which is connected wit-h the plates b and c by pins d.

In the Aexamples of Figs. 4 and 5 glass is material of the intermediate member a, the latter being integral with the plates b and-c.

I claim:`

1. An opt-ical square'consisting of two Asilvered glass plates .and an intermediate member, these three parts having about the same .thermal conductivity-' 2. An optional square consisting of two silvered glass platesand an intermediate glass member.

3. An optical'square consisting of two silyvered glass pla-tes and an intermediate glass 

